Method of preserving mechanical properties of steel wire during spot welding

ABSTRACT

A method for preserving the mechanical properties of steel wire or rod during spot welding of such steel wire or rod to form cages for use in prestressed concrete. The hoop reinforcing wire is, during the winding operation, preheated between a pair of feeders by direct passage of current therethrough to a temperature of from 300*-1000* C. Thereafter the thus heated wire is successively welded to reinforcing rods.

United States Patent Amakasu et al.

[54] METHOD OF PRESERVING MECHANICAL PROPERTIES OF STEEL WIRE DURINGSPOT WELDING [72] Inventors: Tatuo Amakasu, Tokyo; Takao Yamanki,

Fujisawa, both of Japan [73] Assignee: Koushuha-Netsuren KabushlkiKalsha,

Tokyo, Japan [22] Filed: Aug. 3, 1970 [21] Appl. No.: 60,686

Related US. Application Data 4 [62] Division of Ser. No. 781,174, Dec.4, 1968, Pat. No.

[52] US. Cl ..2l9/58 [51] Int. Cl ..B23k 11/00 [58] Field of Search..2l9/56, 57, 58

[ 1 Mar. 7, 1972 [56] References Cited UNITED STATES PATENTS 1,878,7609/1932 Cosgrove et al. ..2l9/56 X Primary Examiner-J V. Truhe AssistantExaminerl'lugh D. .laeger An0meyWenderoth, Lind & Ponack [5 7] ABSTRACTA method for preserving the mechanical properties of steel wire or rodduring spot welding of such steel wire or rod to form cages for use inprestressed concrete. The hoop reinforcing wire is, during the windingoperation, preheated between a pair of feeders by direct passage ofcurrent therethrough to a temperature of from 300l000 C. Thereafter thethus heated wire is successively welded to reinforcing rods.

1 Claims, 6 Drawing Figures Patented March 7, 1972 3,648,006

2 Sheets-Sheet 1 TATUO AMAKASU and TAKAO YAMAZAKI, Inventors 'UmbdZlMAttorneys 2 Sheets-Sheet 2 Po kented Max-( 2h 7, 1972 FIG. 20

TATUO AMAKASU and TAKAO YAMAZAKI, Inventors MMXQM Attorneys O 10 5 O 0 40 O -3 d d O & M m l 9 8 7 6 5 4 3 :GzmEm 562E zoEGzod FIG. 2b

FIG. 4*

METHOD OF PRESERVING MECHANICAL PROPERTIES OF STEEL WIRE DURING SPOTWELDING This application is a division of application, Ser. No. 781,174,filed Dec. 4, 1968, now U.S. Pat. No. 3,573,418.

BACKGROUND OF THE INVENTION The present invention relates to a method ofpreserving the mechanical properties of steel reinforcing wire or rodduring spot welding, by preventing deterioration or recovering the lostmechanical properties of the welded parts of said steel wire or rod.More particularly, this invention relates to a method for preserving, byrecovering, or preventing loss of, mechanical properties of the weldedparts of steelreinforcing wire or rod used in the formation of steelwire cages to be embedded in concrete for the purpose of enhancing thestrength of concrete structural products such as concrete pipes, poles,piles, tanks, etc., of prestressed concrete. Such loss of properties.occurs when spot-welding the contacting parts of steel rods andhoop-reinforcing wires, said steel rods being arranged parallel to eachother and at certain intervals according to the external shape of theconcrete structure, and said hoop-reinforcing wires being fixed spirallyon the outer circumference of said steel rod arrangement.

Hitherto, in manufacturing prestressed concrete structures, the methodwhich has been generally adopted is to arrange the required number ofreinforcing steel rods parallel to each other and at certain intervals,according to the external shape of the structure, such as a concretepillar or the like which will have the rods incorporated therein. Thensteel reinforcing wires or the like are wound spirally around the outercircumference of the steel rod group thus formed. I

To fix and secure the contacting parts of said steel rods and saidhoop-reinforcing wires, they are bound manually with fine steel wires,thus forming a columnar steel wire cage. Said cage is set in theconcrete mold in the proper position, and tension is applied to saidsteel reinforcing rods while concrete is poured into the mold. This isfollowed by introducing the necessary stress into the concrete with thesteel rods and placing the mold in a centrifugal pillar manufacturingmachine or the like to string it up. It is then cured in steam, and themold is removed. It is then cured in water and in air, and a prestressedconcrete pillar is obtained having satisfactory strength and into whichthe stress has been introduced by the steel rods.

An automatic forming machine has recently been introduced to make highlyefficient the production of such a steel reinforcing wire cage that isan essential element for enhancing the strength of such a concreteproduct.

The object of this invention lies mainly in making it possible to formadvantageously and with high efficiency steel wire cages composed ofreinforcing steel rods, which cages have excellent mechanicalproperties.

Other objects and advantages of the invention and its valuablecharacteristics will become apparent from the following descriptiontaken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1a and lb are side and endelevation views, respectively, showing a steel reinforcing wire cage;

FIGS. 2a and 2b are graphs showing changes in the mechanical propertiesof steel reinforcing rods before and after the spot welding of thecontacting parts of the hoop-reinforcing wires to said steel rods; FIG.2a showing the change in tensile strength, and FIG. 2b showing theelongation;

FIG. 3 is a schematic view of one embodiment of heating means forcarrying out the method of the invention; and

FIG. 4 is a schematic end view of a form of steel reinforcing wire cagewhich can be formed successfully by the means shown in FIG. 3.

The above described efficient method of forming steel reinforcing rodcages by using an automatic forming machine will be illustrated withreference to FIG. 1. The required number of steel reinforcing rods 1 arearranged parallel to each other with their ends fixed to a holding meanssuch as an anchor plate 5. At spaced points along the rods there ispositioned within the inner circumference of the rods a drum 4 .or thelike. The group of steel rods can move and revolve continuously alongthe outer circumference ofsaid drum 4. Around the outer circumference ofthe group of steel rods isspirally wound hoop-reinforcing wires 2 madeof steel. This isdone .by relatively rotating the rods and the wire,.i.e., by fixing the group of steel rods and rotating thehoop-reinforcing wires 2, or vice versa The points 3, where therespective steel rods 1 and the hoop-reinforcing wires 2 contact eachother, are successively and automatically spot-welded by an electrode 6for spot welding,.which electrode is connected to a power source Therods generally are of a steel having a carbon content of from '0.20.65.Typical compositions fall within the ranges of Table I. TABLE I c l siMn I I P s 0.20.6S 0.154.! 0.60-0.90 o.oso o.oss

Typical compositions of the hoop-reinforcing wires are less In so far asformation of this kind of steel wire cage by this method is extremelyefficient, said method is superior in comparison to conventionalmethods. However, a serious shortcoming of said method is that, when therods 1 and wire 2 are welded, the mechanical properties of the weldedparts of the rods deteriorate because said welded parts of the steelrods are suddenly heated and suddenly cooled off.

According to the experimental results obtained by applicants in thisrespect, the values of the tensile strength and elongation of steelreinforcing rods 1 which have not been spot welded are as indicated bydots c in FIGS. 2a and 2b, respectively, while the values of the tensilestrength and elongation of the same steel rods measured after thehoop-reinforcing wires 2 are mechanically separated from the steel rods1 after having been spot welded thereto is indicated by dots d in thesame Figs. As can be seen, the mechanical properties of the steelreinforcing rods 1 will be changed so that the tensile strength will bereduced about 2 percent, and the elongation will be reduced about 50percent. These reductions were substantially similar to those obtainedin other experiments of the same kind. According to the results ofapplicants research and experiments, this can be attributed to the factthat a partial metallographical-structural change is brought about inthe welded parts of the steel reinforcing rod which contains carbon inan amount up to the level of medium carbon steel. The change in themetallographical structure of the welded portion is due to the fact thatit is heated suddenly and quickly by the heat generated during spotwelding, and thereafter is suddenly and quickly cooled off. A seriousconsequence of such a structural change is that it will, in its turn,adversely influence the full achievement of the purpose of this kind ofsteel wire cage by failure to introduce sufficient stress into theconcrete structure to enhance the strength thereof.

After all of the various experiments and studies carried out in anattempt to find a solution to this difficulty, applicants have finallydiscovered that the hoop-reinforcing wire in the winding operationshould be preheated between a pair of feeders by direct passage ofcurrent therethrough to a temperature of from 300 to 1000" C., and thenthe thus heated wire should be welded to the reinforcing rods atsuccessive spots therealong. By this means, deterioration of themechanical properties in said welded parts can be effectively prevented.

In FIG. 3 there is schematically shown an apparatus for carrying out themethod by direct electrification of the hoop-reinforcing wire, and inwhich the steel reinforcing rods 1 are surrounded by hoop-reinforcingwire 2. An electrode 6 for spot welding bears on the wire 2 and alsoserves to press the hoopreinforcing wires against steel rods 1. Atransformer TH for direct electrification has one end of its secondarywinding connected electrically to a feeder 7 which is nearer the hoopreinforcing wire 2 and which is shiftable along said hoop-reinforcingwire 2 and has the other end connected electrically to another feeder 7'having a similar construction. A transformer TW for spot welding has oneend of the secondary winding connected electrically to the electrode 6and the other end connected to the drum 4.

In operation, current from the transformer TI-I for directelectrification is passed through the feeders 7 and 7' to thehoop-reinforcing wire 2 prior to spot welding them to the steel rods 1and the hoop-reinforcing wire 2, the current flowing from the feeder 7through the hoop-reinforcing wire 2 to the feeder 7. As a result, it ispossible to heat the portion of the hoop-reinforcing wire between thefeeders 7 and 7 to the required temperature of from 300 to l,000 C. Inthis way, the hoop-reinforcing wire 2 heated to the required temperaturewill be wound automatically around the circumference of the steel rodsby rotation of said steel rods in the direction of the arrow in thedrawing and longitudinal movement, either of the rods 1 or the feeders 7and 7'. The steel rods 1 and the heated hoop-reinforcing wire 2 are spotwelded by the electrode 6 where they contact each other.

Experiments show that in the method of the invention, when thehoop-reinforcing wire has been heated to a temperature of from 300 tol,000 C. before the parts of the steel rods and the hoop-reinforcingwire, which contact each other, are spotwelded, deterioration of themechanical properties of the welded part is only partial and small, andthat such insignificant deterioration is rapidly overcome and the partsreturned to normalcy by virtue of the heat retained by thehoop-reinforcing wire.

EXPERIMENTAL RESULTS TABLE III Preheating of Elonga- Hoop-ReinforcingTensile tion Breaking Wire Load Position 9380 3.8 Spot welded part.No-heating 9400 6.3 Spot welded part. 9500 6.4 Spot welded part.

Heating Temperature 9500 6.9 Spot welded part. About 200C 9550 5.6 Spotwelded part. 9500 7.5 Spot welded part.

Heating Temperature 9570 8.9 Parallel pan. About 300 C 9480 8.3 Parallelpart. 9500 8.3 Parallel part.

9520 8.8 Parallel pan. About 400 C 9530 8.6 Parallel part. 9500 9.0Parallel part.

9480 9.8 Parallel part. 900 to l000c 9500 9.0 Parallel part. 9480 9.2Parallel part.

Raw Material (Steel 9500 9.8 9550 9.4

Reinforcing Rods) Table 3 shows a part of the experimental resultsachieved by using the method of direct electrification, and shows acomare heated to various heating temperatures before spot weldslxqsa stis y- From the above table it is clear that, if the heating temperaturesprior to spot welding are from 300 to 1,000 C., the effect of heatingthe'hoop-reinforcing wires 2 is marked, there being substantially nodifference between so-called raw material steel reinforcing rods whichhave undergone no spot welding so far as tensile strength, elongationand breaking position are concerned. In addition, another effect of themethod using heating of the hoop-reinforcing wires is that it ispossible to reduce the plastic deformation resistance of thehoop-reinforcing wires 2 by heating them to a temperature of from 300 to1,000" C. before spot welding, and therefore, particularly when steelwire cages of small diameter are to be formed, the conventionaloperational difficulty heretofore suffered due to the fact thathoop-reinforcing wires are not easily plastically deformed during thewinding round the circumference of the steel rods can be eliminated. Itis therefore possible, for example, to form a steel wire cage having asquare cross section, as shown in FIG. 4.

Further, it should be noted that the system illustrated in FIG. 3provides a method for very efficiently preventing deterioration ofmechanical properties in steel rods without around the periphery of agroup of steel rods in the cage-formthe hoop-reinforcing wire 2, whichis being moved for winding ing process, is heated to a specifiedtemperature between the two feeders 7 and 7, and said hoop-reinforcingwire maintained at said specified temperature is spotwelded to said rodsduring movement. Thus, in the process of cage formation, spot welding ofwire to rods as well as prevention of deterioration of mechanicalproperties'in steel rods can be finished in a single step without anyloss of efficiency and with a negligible loss of energy. On the otherhand, in a conventional cage-forming process, it is common practice towind a hoop-reinforcing wire around the steel rods while shifting theformer at a high speed of 20-100 cm./sec. In such a method, when heatingthe hoopreinforcing wire in motion with a flame, the energy isinsufficient. When induction heating is used, the equipment cost rises.Thus, both methods are unsatisfactory from a practical standpoint. Incontrast thereto, direct electrification, as illustrated in FIG. 3, canheat the hoop-reinforcing wire to the specified temperature simply andefiiciently while the rods are moved at such high speed before spotwelding.

What is claimed is:

l. A method of preventing deterioration of the mechanical properties ofthe welded parts of steel reinforcing rods due to spot welding duringformation of steel wire cages for manufac-' turing prestressed concretestructures, comprising feeding hoop-reinforcing wire through a pair offeeders and winding it around a series of parallel reinforcing rodsarranged in a cylinder to carry out a continuous cage winding operationprior to its being spot welded to the reinforcing rods, feeding a directcurrent from said feeders through the hoop-reinforcing wire between thefeeders for heating the wire to a temperature of from 300 to l,000 C.,the wire being substantially immediately contacted with a reinforcingrod after it leaves the feeder closet to the reinforcing rod, and thenwelding the thus heated wire to the reinforcing rods at successive spotstherealong, whereby the wire is welded while it is still substantiallyat the heated temperature.

1. A method of preventing deterioration of the mechanical properties ofthe welded parts of steel reinforcing rods due to spot welding duringformation of steel wire cages for manufacturing prestressed concretestructures, comprising feeding hoop-reinforcing wire through a pair offeeders and winding it around a series of parallel reinforcing rodsarranged in a cylinder to carry out a continuous cage winding operationprior to its being spot welded to the reinforcing rods, feeding a directcurrent from said feeders through the hoop-reinforcing wire between thefeeders for heating the wire to a temperature of from 300* to 1,000* C.,the wire being substantially immediately contacted with a reinforcingrod after it leaves the feeder closet to the reinforcing rod, and thenwelding the thus heated wire to the reinforcing rods at successive spotstherealong, whereby the wire is welded while it is still substantiallyat the heated temperature.